A dog kidney and liver cytosol protein which binds thyroxine (T4) and triiodo-thyronine (T3) is to be subjected to further purification and to undergo additional studies of its physical properties; possible functional roles for this high affinity/high binding capacity cytosol thyronine-binding protein (CTBP) are to be investigated. Purification of the acidic protein is to be effected by sequential preparative isoelectric fractionation (ampholyte pH 2.5-6.0) of cytosol (105,000 X g supernatant), gel filtration and preparative polyacrylamide gel electrophoresis. Physical characteristics to be defined include binding site specificity (to be measured in terms of affinity of CTBP for analogues of thyroid hormone), nature of the tendency of CTBP, like certain sex steroid "receptor" proteins, to aggregate, and amino acid composition of purified material. Avidity of the high- and low- affinity hormone-binding sites on CTBP for analogues is measured by Scatchard analysis of bound/free data developed by charcoal adsorption of hormone-containing cytosols. Aggregation is studied by estimating influence on this phenomenon of agents such as glycerol, Mg and Ca, and by analyzing behavior of native and aggregate CTBP on ion exchange chromatography. Possible functions of CTBP are to be studied by assessment of the influence of aggregated and nonaggregated CTBP on nuclear uptake of T4 and T3, ability of CTBP to control mitochondrial uptake of T4 and T3 (and to influence mitochondrial protein synthesis), and the role of CTBP in control of exchangeable cellular thyroid hormone. The latter is to be studied by means of isolated liver perfusion. The responsiveness of concentration in liver and kidney of CTBP to circulating levels of thyroid hormone, estrogen and androgen is to be quantitated. In such studies binding site number in cytosol is measured by charcoal adsorption partition of bound/free hormone and Scatchard plot.